Rubbery materials are usually electric insulators. A number of conductive rubbers are available as mixtures of rubbery material and conductive agents. For example, rubbers having carbon black blended therein to provide an electric resistivity of from 10.sup.-1 to 10.sup.2 .OMEGA..multidot.cm are conductive rubbers which are used in various applications.
Silicone rubber is also widely used as electrically insulating rubber because of its heat resistance, low-temperature resistance and weatherability. It can also be used as conductive silicone rubber by adding conductive agents like the other rubbery materials.
The conductive agents which are added to silicone rubber for imparting electric conductivity are typically carbon black, graphite, various metal powders such as silver, nickel, and copper, various non-conductive particles and monofilaments surface treated with silver or similar metals, carbon fibers, and metallic fibers. By mixing these conductive agents, the electric resistivity of silicone rubber can be reduced to the order of 10.sup.10 to 10.sup.-3 .OMEGA..multidot.cm depending on the type and amount of conductive agent without detracting from the inherent properties of silicone rubber.
However, silicone rubbers having conductive carbon black such as Ketjen black and acetylene black blended therein show a substantial variation of electric resistivity in the semiconductive range of from 10.sup.3 to 10.sup.10 .OMEGA..multidot.cm. It is difficult to maintain the electric resistivity of carbon black filled silicone rubbers stable probably because the dispersion of carbon substantially varies with molding conditions.
A variety of rubbery parts are used in business machines. These rubbery members represent a demand for rubbery material having electric resistivity in the semiconductive range. Therefore, there is a need for rubbery material exhibiting a minimal variation of electric resistivity in the semiconductive range, that is, a stable electric resistivity.